In fuel cells, a layer containing a catalyst for electrode (hereinafter, also the electrocatalyst) is usually provided on the surface of a cathode (air electrode) or an anode (fuel electrode). (Such layers are also referred to as the catalyst layers hereinafter.)
Typical electrocatalysts for fuel cells are platinum catalysts that are stable at high potential and have high catalytic activity. However, since platinum is expensive and exists in a limited amount, alternative catalysts have been desired.
Metal oxide electrocatalysts attract attention as cathode catalysts alternative to the platinum catalysts. Metal oxides are generally stable and are not corroded in acidic electrolytes or at high potential. Further, metal oxide catalyst layers formed on the surface of electrodes stabilize the structure of the electrodes.
For example, Patent Document 1 (JP-A-2004-95263) discloses fuel cell catalysts containing a metal oxide such as WO3, TiO2, ZrO2, PtO, Sb2O4 or Sb2O3 as electrocatalysts. However, the fuel cell catalysts also involve platinum and still have the problems as described above.
Patent Document 2 (JP-A-2005-63677) discloses fuel cells that have an electrocatalyst selected from ruthenium oxide, titanium oxide, vanadium oxide, manganese oxide, cobalt oxide, nickel oxide and tungsten oxide. However, these metal oxides as electrocatalysts show low oxygen reduction activity.    Patent Document 1: JP-A-2004-95263    Patent Document 2: JP-A-2005-63677